This invention relates to methods and apparatus for accurate advancement of media in a printer or other recording device.
One type of ink-jet printer includes at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a printhead that is mounted to the body of the cartridge. The printhead is controlled for ejecting minute drops of ink from the printhead to a sheet of print medium, such as paper, that is advanced through the printer.
The printer includes a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the drops onto the paper is controlled to form a swath of an image with each scan. The height of the printed swath (as measured in the direction the media is advanced) is fixed for a particular printhead.
Between carriage scans, the media is advanced so that the next swath of the image may be printed. In most cases, the base of the just-printed swath must be precisely aligned with the top of the next-printed swath so that a continuous image may be printed on the paper. Alternatively, the paper may be advanced by less that a full swath height to effect xe2x80x9cshinglingxe2x80x9d type of printing. In any event, inaccurate media advances between scans of the carriage result in print quality artifacts known as banding.
The prevention of banding artifacts thus calls for precise control of the advancing media in discrete steps between printed swaths. The demand for accuracy in advancing media becomes greater as printhead development leads to higher and higher resolutions, thereby reducing the tolerances permitted in advancing the media.
Rotary optical encoders with associated servo systems are commonly used in printers for accurately advancing print media between carriage scans. The encoder is connected to a media advance mechanism of the printer (drive motor, drive roller, etc.) and its output signals provide the microprocessor-based printer controller with an indication of the position of the media as the media is advanced through the printer. The controller, in turn, controls the drive motor as needed to incrementally advance the media.
The encoder is not located in direct contact with the print media. Rather, the encoder is connected to the drive roller or other mechanism as mentioned above. As a result, the encoder position only indirectly reflects the actual position of the media. Moreover, a rotary encoder, as well as the media drive roller, is susceptible to runout errors. As is known in the art, runout errors are sinusoidally varying errors that occur as a result of slight variations in the concentricity of a mechanism. For instance, a runout error attributable to a drive roller arises when the outer surface of a drive roller is not precisely concentric with the axis about which that roller rotates.
As a consequence of runout errors, the magnitude of the media position changes as represented by the encoder output signals will not precisely match the actual position change of the media. The errors attributable to encoder and drive roller runout will combine into a single characteristic sinusoidal feed error for that particular printer. It is this overall error that must be accounted for in order to accurately advance the media in the printer.
The recognition of runout errors and the general notion of accounting for such errors have produced a few solutions. For example, one can employ a second rotary encoder that is mounted 180xc2x0 out of phase with the primary encoder. The combined output of both encoders has the effect of averaging-out the runout errors of the encoders. This approach, however, does not account for runout errors of the drive roller or other associated rotating media-advance components that are between the encoders and the print media. The provision of a second encoder also adds significant cost to the system.
Another approach to addressing runout errors (as described in U.S. Pat. No. 5,825,378) is to draw a series of lines on media using a swath-type printer. The lines correspond to an angle of rotation of the drive roller or platen that carries the media. A carriage-mounted optical sensor thereafter reads the actual position of the lines, and this position information is used to generate a correction signal. This approach, however, is limited by the accuracy of the encoder system that is used with the carriage drive, as well as unrelated dot-placement errors associated with ink-jet printers.
The present invention is directed to a simple, yet accurate way of determining calibration values for correcting the characteristic sinusoidal feed errors of a printer or other recording device (such as a fax machine, plotter, etc.).
In the preferred embodiment of the invention, a sheet of calibration media is employed for facilitating the calculation of the calibration values. The sheet carries a number of targets and is used in a way that prevents the calibration media errors from affecting the calculation. The term xe2x80x9ccalibration media errorsxe2x80x9d generally means the errors or deviations between the measured, nominal locations of the targets and the actual locations of the targets on the sheet resulting from inaccuracies in measurement of those targets, which would otherwise introduce additional errors, and thus defeat the calibration process.
As will be described below, the calibration media is fed twice through the printer, and target-position data is collected each time. According to the present invention, the position data is processed in a way that cancels the attendant calibration media errors so that the calculated calibration values precisely correct the characteristic sinusoidal feed error of that printer.
Inasmuch as the errors associated with the calibration media are cancelled, the approach of the present invention dramatically reduces the precision (hence, cost) with which the calibration media need be prepared. This, in turn, makes it possible to generate the sheet of calibration media, at any time desired. One can even use the printer being calibrated for generating the calibration sheet.
Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.